1. Field of the Invention
This invention relates to a method for preparing a positive electrode active material for a non-aqueous secondary battery.
2. Description of Related Art
In a non-aqueous secondary battery, a positive electrode active material is used.
With a rapid progress in the field of electronic instruments toward portable and/or cordless ones, development of non-aqueous secondary batteries which can realize the desire for batteries which are smaller in size, lighter in weight and have higher capacity than previous secondary batteries has been forwarded. Among them, lithium secondary batteries are already used in practice as power sources for portable telephones, note-book sized personal computers, and the like and, further, investigation is being made to attain a larger size and higher output batteries to be used as power sources for automobiles and for communication power backup.
For a positive electrode active material for non-aqueous secondary batteries, there has hitherto been used, for example, spinal type lithium manganese oxide; but a positive electrode active material which can provide a non-aqueous secondary battery having a higher capacity has been eagerly awaited.
Under such situations, novel compounds or the like which are compounds comprising lithium, nickel and manganese and having a layered structure and are represented by the composition formula LiCo1/3Ni1/3Mn1/3O2 (Chemistry Letters, Chem. Soc. Jpn., July 2001, p. 642) or LiNi1/2Mn1/2O2 (Chemistry Letters, Chem. Soc. Jpn., August 2001, p. 744) or by the formula Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 (O≦x≦1/2) or Li[NixCo1-2xMnx]O2 (0<x≦1/2) (preliminary papers for 42nd battery symposium, Nov. 21, 2001, lecture no. 2I12) have been proposed as a positive electrode active material for a non-aqueous secondary battery which can possibly solve the above-mentioned problems and have been attracting attention. The “layered structure” herein refers to a structure wherein the crystal structure is identified as the α-NaFeO2 type by X-ray diffraction (Electrochemical and Solid-State Letters (USA), Electrochemical Society, Inc., December 2001, vol. 4, p. A200-A203).
As the nickel source and manganese source for synthesizing these compounds, since it has previously been considered that the above-mentioned compounds having a layered structure cannot be obtained by using a nickel compound and a manganese compound as a mixture thereof, there have been used complex hydroxides of nickel with manganese. However, since divalent manganese in the comlex hydroxide is easily oxidized into trivalent state, the conditions of synthesis of the complex hydroxide and the atmosphere in which subsequent handling thereof is conducted both need to be controlled strictly and hence the complex hydroxide has been difficult to prepare. Accordingly, a simple and easy method for preparation which does not need strict control of atmosphere has been eagerly awaited.
The object of this invention is to provide a simple and easy method for preparing a positive electrode active material for a non-aqueous secondary battery which comprises a compound comprising lithium, nickel and manganese and having a layered structure, a positive electrode active material for a non-aqueous secondary battery obtained by the method, and a non-aqueous secondary battery which uses the positive electrode active material.
The present inventors have made extensive study on the method for preparing a positive electrode material for a non-aqueous secondary battery which comprises a compound comprising lithium, nickel and manganese and having a layered structure by firing a mixture which is a mixture of metal compounds and comprises lithium, nickel and manganese. As the result, the inventors have found that when a mixture furhter containing at least one member selected from the group consisting of dinickel trioxide and boron compounds is used, the intended positive electrode active material can be prepared in a simple and easy way, without requiring strict control of atmosphere, by mere firing. This invention has been completed on the basis of above finding.
Thus, according to this invention, there is provided a method for preparation of a positive electrode active material for a non-aqueous secondary battery comprising a step of firing a mixture of (1) at least one member selected from the group consisting of dinickel trioxide and boron compounds and (2) one or more metal compounds comprising lithium, nickel and manganese as their metal elements, and a step of obtaining a compound comprising lithium, nickel and manganese and having a layered structure.